1. Prior Art
______________________________________ Banczak 4,021,252 May, 1977 Wachtel 4,024,096 May, 1977 Parkinson 4,045,397 Aug. 1977 Hwang 4,070,322 Jan. 1978 Keating 2,356,065 Aug. 1944 Zabiak 3,687,887 Aug. 1972 Zabiak 3,705,043 Dec. 1972 Ostergren 3,846,141 Nov. 1974 Miyajima 3,912,520 Oct. 1975 Hertz 3,994,736 Nov. 1976 Sambucetti 4,026,713 May, 1977 ______________________________________
Edds et al., "Smear Resistant Jet Ink," IBM Technical Disclosure Bulletin, Vol. 16, No. 6, November 1973.
2. Field of Invention
Metal cans constitute a very widely utilized medium for the protective packaging of a great variety of products, many millions of cans being used daily for packaging of foods, beverages and many other materials. For many years, the common metal can was constructed of tinplated steel, and was customarily referred to in the industry as an ETP can, the initials standing for "electro-tin-plated." Containers of this type are referred to by the public at large as "tin cans." In recent years, however, the metal can industry has developed organic polymeric resinous coatings for metal cans which offer substantially the same protection to the steel as the traditional coating of metallic tin. These organic resin based coatings have been applied both as a top coat over the traditional thin tin coating and as the sole protective coating composition applied directly to the steel can body and/or end components to yield both resin coated ETP cans and resin coated untinned steel cans which are now referred to in the industry as "tin-free steel" or TFS cans.
Manufacturing and processing concerns which package various products in metal cans have found it highly desirable to print, at some point on the can surface, a series of coded symbols which carry information of interest primarily to the packager, including the particular machine on which the can was packed, the date and time of packing and perhaps even the identity of the machine operator. Such data are useful in case it is necessary to trace any particular can or cans after they have been packed.
Many of the products packaged in metal cans are subjected to conditions of high temperature and high moisture during pasteurization or sterilization procedures carried out before or after the can is filled with product and sealed. In order to be commercially satisfactory, the coded indicia printed on the cans must be capable of withstanding these processing conditions as well as being resistant to rubbing abrasion.
This invention is directed primarily to ink compositions suitable for printing identifying indicia on the resin coated surfaces of TFS, ETP and coated aluminum cans and can components as well as such metals free of organic resins by means of ink jet printing techniques. In a printing apparatus operating on the so-called "ink jet printing" principle, in general terms, a fluid ink is forced, under pressure, through a very small orifice in an orifice block which contains a piezoelectric crystal vibrating at high frequency (50-100,000 vibrations per second) causing the ink passing through the orifice to be broken into minute droplets equal in number to the crystal vibrations. The minute droplets are passed through a charging area where individual droplets receive an electrical charge in response to an electronic signal, the amplitude of the charge being dependent on the amplitude of the signal. The droplets then pass through an electrical field of fixed intensity, causing a varied deflection of the individual droplets dependent on the intensity of the charge associated therewith, after which the deflected drops are allowed to impinge on the base medium which is to receive the decorative or informative printed indicia. Apparatus suitable for carrying out the ink jet printing process is described in detail in U.S. Pat. Nos. 3,465,350 and 3,465,351, issued Sept. 2, 1969, and it is in connection with an apparatus and process as described in the aforementioned patents that the ink of the present invention is designed to function.
In order to operate satisfactorily in an ink jet printing system, an ink must display a consistent breakup length, drop velocity and drop charge under set machine operating conditions. To achieve these ends, the ink must meet strict requirements with regard to viscosity and resistivity, solubility and compatibility of components, stability and anti-skinning properties and must readily re-dissolve in a suitable solvent for rapid cleanup of the machine components with a minimum of effort. In order to facilitate cleanup of the apparatus after use, the ink components should be readily soluble in a common solvent medium. This will prevent any gradual buildup of ink residues in the system which could result in malfunction.
The ink properties set forth above are primarily established by the requirements of the jet printing apparatus. In addition to these requirements, the ink must possess certain other properties which are specifically related to its intended use in the printing of metal cans and, in particular, can bodies intended for the packaging of foodstuffs and beverages.
For example, the ink must properly wet the can surface, whether coated or uncoated, on which the printed indicia are to appear. If the ink is of such composition that it fails to readily wet the coated metal surface, the ink will bead up on the surface and fail to adhere properly to it. In extreme cases, the beaded drops will coalesce into larger droplets which run and make the printing completely unintelligible. The problem is particularly accentuated by oily or greasy residues left on the can surface from earlier stages of fabrication of the container. On the other hand, if the ink is of such composition as to wet the coated metal surface too readily, the ink drops will flatten out and spread excessively on the surface, diluting the color intensity of the ink and overlapping the image of adjacent dots making the printed image fuzzy and the characters unintelligible. In addition to the requirement of proper wetting of the surface to be printed, the droplets of ink must adhere strongly to the surface after application and drying so that the printed matter is resistant to both physical rubbing or abrasive action and also is resistant to moisture. The ability of the ink to form and retain a desired image on a TFS or coated ETP or aluminum can surface in the presence of moisture and the ability to resist removal by moisture is of great importance in this application because the metal can surfaces are generally damp before, during and after the printing operation. It is particularly difficult to maintain satisfactory adhesion of the ink to metal cans which are subjected to pasteurization, the combination of moisture and high temperature utlized in this process tending to cause the coloring matter to bleed, and to severely reduce the adhesion of the ink to the can body so that it is readily removed by subsequent rubbing or abrasion.
Several attempts have heretofore been made to provide jet ink compositions meeting all of the above requirements and overcoming the difficulties enumerated. One example is U.S. Pat. No. 4,021,252 issued May 3, 1977 to Daniel Philip Banczak and William Eric Tan and commonly assigned herewith, said patent providing excellent inks of the binder type exhibiting the above discussed properties and which are particularly suitable for use on coated and uncoated aluminum cans. Representative inks therein comprise a colorant, a shellac binder, a glycol solvent binder and a water-alcohol blend.
Other examples are inks described and claimed in application Ser. No. 634,507 filed Nov. 24, 1975 of R. L. Germonprez, also commonly assigned herewith. The inks of said application are of the binderless type comprising a colorant and a homogeneous blend of water, a lower aliphatic alcohol, an oxygenated aliphatic or cyclic ketone, a surfactant and an aliphatic hydrocarbon. The ink compositions are designed primarily for use on coated TFS or ETP metal cans and comprise a solvent system so selected as to soften and swell the organic polymer substrate sufficiently to allow penetration of the sub-surfaces thereof whereby the indicia printed on the substrate become highly resistant to abrasion and sterilization process. In said compositions, the function of the aliphatic hydrocarbon is to penetrate the thin layer of oily material which serves as a lubricant on the resin coated metal surfaces of can components in the can forming operations.
The present invention represents an improvement over the inks referred to above. It has been found that the presence of tetrahydrofuran in such compositions conveys unexpectedly improved characteristics in the ability of the ink composition to penetrate varied types of organic coatings and lubricants and at the same time improves the ability of both binder and binderless types of ink compositions to properly wet a larger number of substrates including coated and uncoated aluminum or steel. Additionally, such improvements are readily observed whether water is present or absent as a component of the composition. Solvent volatility is likewise improved.